1. Field of the Invention
The present invention relates to Ni based alloys, and it particularly relates to Ni based alloys for forging having excellent high temperature strength and oxidation resistance.
2. Description of Related Art
In order to improve the power generation efficiency of generators such as steam and gas turbine generators, it is effective to raise the main steam temperature or combustion temperature. When the main steam (or combustion) temperature of a generator is increased, the temperatures of the generator components also rise. Such components used at higher temperatures than conventional ones require to be made of materials having a higher maximum allowable use temperature.
Materials for high temperature components are classified into those for precision casting and these for forging, depending on the use temperature and the component size. Small components used at high temperatures (such as stator vanes and rotor blades of a gas turbine) are usually formed by precision casting. On the other hand, large components are usually formed by forging because they are difficult to precision cast. Forging materials are generally hot forged in the temperature range of 1000 to 1200° C., and therefore desirably have low deformation resistance above 1000° C. to ensure workability.
Nickel (Ni) based superalloys strengthened by γ′ phase (Ni3Al) precipitation have excellent high temperature strength, and are therefore widely used for forging high temperature components. However, the presence of γ′ phase precipitates in the superalloy reduces hot workability. The γ′ phase is stable at lower temperatures and dissolves into a matrix above a threshold temperature. Therefore, hot working is usually performed above the temperature of solid solution limit line (solvus temperature) of the γ′ phase (a threshold temperature at which γ′ phase precipitates disappear).
The larger the amount of γ′ phase precipitation in an alloy is, the higher the strength of the alloy is; so it is desirable to increase the amount of γ′ phase precipitation at the use temperatures of the alloy. However, increase in the amount of γ′ phase precipitation will result in an increase in the temperature of solid solution limit line (solvus temperature) of the γ′ phase, thus reducing the hot workability. This has hitherto prevented any significant improvement in the high temperature strength of forging materials strengthened by γ′ phase precipitation.
Generally, high temperature components are required to have a 100,000-hour creep rupture strength of 100 MPa at their use temperatures. In conventional materials, it has been necessary that the temperature of solid solution limit line of the γ′ phase of a forging alloy is suppressed to 1000° C. or lower in order to ensure sufficient hot workability, the allowable use temperatures of the alloy, at which the above-mentioned strength requirement is satisfied, is limited to 750° C. or lower.
In addition, such alloys are significantly oxidized above 750° C. Therefore, it is also essential to increase the oxidation resistance of an alloy in order to increase the maximum allowable use temperature to higher than 750° C. In order to increase the oxidation resistance of an alloy, it is effective to add aluminum (Al) to the alloy since oxides of Al are stable. However, addition of Al to an alloy increases the temperature of solid solution limit line of the γ′ phase and reduces the hot workability. Because of this, in conventional forging alloys, the Al content is limited to 3 wt. % or less, which is insufficient for stably forming oxides of Al.
Furthermore, according to conventional knowledge, it is also essential to add niobium (Nb), titanium (Ti) and tantalate (Ta) to conventional Ni based forging alloys in order to stabilize the γ′ phase at higher temperatures and increase the strength (see JP-A-2005-97650). However, for forging alloys strengthened by γ′ phase precipitation, prior arts cannot simultaneously achieve sufficient hot workability and sufficient high temperature strength.